This invention describes a new method of constructing a tunable Fabry-Perot interferometer which results in a rugged, highly stable, low optical loss, self temperature compensating and dynamically re-configurable tunable filter for a number of applications including fiber optic communication systems.
The tunable optical filter system consists of an optical module having a one or two-piece cylindrical housing. A thin flexible diaphragm using the same material as the housing is either an integral part of one of the housing""s two segments, as shown in FIG. 2 or bonded to the housing as shown in FIG. 6. Two piezoelectric rings are co-axially attached to the faces of the diaphragm. The two piezo elements, when properly connected electrically, form a bimorph. Under an applied voltage or pathlength control signal, the bimorph flexes the diaphragm and causes an axial motion to any object attached to it.
Input and output fibers are each co-axially attached to one side of a collimating lens. Each lens collimates the beam passing through it as the beam exits the lens. The lens can be made an integral part of the fiber by appropriately shaping the end or the lens can be a separate element to which the fiber is attached using conventional techniques such as epoxy bonding or laser fusion. If a separate lens is used, the preferred approach is to deposit anti-reflection coating on the two contacting surfaces to reduce the filter""s optical losses and back reflections. Graded index (GRIN) lenses are particularly useful for this purpose. As is well known, a GRIN lens with a xc2xc pitch at the desired wavelength converts the diverging output of an attached fiber to a parallel beam. The fibers can be single-mode fibers generally used in modern fiber optic networks. The other sides of the two collimating pigtailed lenses, when coated for high reflectivity, form the two reflecting surfaces of the Fabry-Perot interferometer, as shown in FIG. 2.
The two lenses have flat mirror surfaces that partially transmit light passing through them. The reflecting surfaces are placed parallel to each other within the optical module. The lenses are positioned by attaching one lens to one segment of the housing and passing its pigtailed fiber through an axial hole in the housing. The other lens is similarly attached to the diaphragm. Such a parallel plate interferometer constitutes a band-pass filter. The distance between the mirror surfaces can be adjusted to pass a particular wavelength and reject others. An electrical voltage applied to piezos flexes the diaphragm which causes the attached lens to move axially thereby tuning the filter to select a particular wavelength. Initial tuning can also be accomplished by manually adjusting a set screw, threaded cap or threaded plug that is in contact with the diaphragm. Another function of the set screw is to adjust the gap between the two reflecting surfaces to a predetermined value during the assembly to account for the dimensional tolerances of the housing and the lenses. The cap or plug has a hole or aperture through it to allow the output fiber to exit the housing or to allow the input fiber to enter the housing.
The tunable filter system also has a coupler. The output fiber is connected to a coupler which uses a small portion of the output power to activate the servo loop for locking the filter""s bandpass to the desired channel. A method to accomplish this is to apply a small amplitude dither voltage to the piezos. If the filter""s bandpass is not centered at the selected channel wavelength, the detector output will be different in the two halves of the dither cycle. The difference is integrated by the electronics and a DC signal is generated which drives the piezo and adjusts the cavity length to pass the desired channel. A separate DC signal can also be superimposed to initially move the bandpass to any desired channel. This latter signal can be programmed and remotely controlled for dynamic reconfiguration of the filter""s bandpass.
Tunable filters are sensitive to temperature changes. An important feature of the present invention is its self-compensating of errors caused by temperature variations.
In a first embodiment, we have a tunable optical filter system comprising; an optical module having, a tunable optical cavity. The optical module responds to a pathlength control signal to select and output a predetermined wavelength signal selected from a optical signal having at least two wavelengths. A servo controller responds to an input signal that adjusts the pathlength control signal to select the predetermined wavelength signal. The servo controller also responds to a sample portion of the predetermined wavelength signal to finely adjust the pathlength control signal to maximize the optical power in the sample portion predetermined wavelength signal. The tunable optical filter system has an optical module that has a frame, and a first and a second axially aligned collimating lens. Each collimating lens has a mirrored surface normal to the optical axis and separated by a gap distance to form a cavity.
The first collimating lens has an input surface coupled to receive the optical signal having at least two wavelengths. The lens collimates the light beam and outputs the collimated beam into the cavity. A diaphragm is coupled to the frame. The diaphragm has at least a first piezo element mechanically attached to the diaphragm to warp the diaphragm in response to the pathlength control signal. The second collimating lens is mechanically coupled to the diaphragm and is axially positioned by the diaphragm to adjust the gap distance to select a predetermined wavelength signal from the collimated beam and to output the predetermined wavelength signal from an output surface.
In another more particular embodiment, the optical module has a frame. An input fiber carries a optical signal from a light source. The optical signal has at least a first and a second wavelength from which a predetermined wavelength is to be selected. The light beam is coupled to a first and a second axially aligned collimating lens. Each collimating lens has a mirrored surface normal to the optical axis. The mirrored surfaces are separated by a gap distance to form a resonant cavity. The input fiber couples the light beam into the first collimating lens. The first collimating lens collimates the light beam and outputs the collimated output beam via the its mirrored output surface into the gap.
In yet another embodiment the optical module has a first cylindrical cup with a rim around an aperture at a first end and a thin base forming a first diaphragm at the second end. The first diaphragm has an inner surface at the interior base of the cup and an outer surface. The first diaphragm has a small centered aperture. A second cylindrical cup has a rim around an aperture at a first end and a base with a threaded aperture at the other end. A cylindrical plug has a threaded portion for insertion in and engagement with the second cylindrical base threaded aperture. The cylindrical plug has a centered aperture. A second diaphragm has a first face and a second face. The second diaphragm is rigidly positioned between the first cylindrical cup rim and the second cylindrical cup rim with the first face facing into the first cylindrical cup aperture. The second diaphragm has a center aperture.
An input fiber passes through the cylindrical plug center aperture and through the second diaphragm center aperture carrying the optical signal from a light source. The optical signal has at least a first and a second wavelength from which a predetermined wavelength is to be selected. The first collimating lens input surface is coupled to the second diaphragm first face. The input fiber has an output end coupled to the first collimating lens input surface to couple the light beam into the first collimating lens. The second collimating lens output surface is coupled to the first diaphragm inner surface. A means for coupling the plug to the second diaphragm second surface is included. The plug is rotated to advance and axially position the first collimating lens mirror surface to a predetermined gap distance from the second collimating lens mirror surface. The tunable optical filter system also has a coupler with an input port coupled via the first diaphragm aperture to the first collimating lens output surface to receive the predetermined wavelength portion of the collimated output beam and to provide a sample portion of the predetermined wavelength signal via its sample output port. A servo controller responds to a wavelength select signal characterized to identify the predetermined wavelength to be selected and provides a coarse pathlength control signal to the piezo elements to adjust the gap distance to coarse select the predetermined wavelength signal. The servo controller also responds to the sample portion of the predetermined wavelength portion of the collimated output beam to adjust the pathlength control signal to the piezo elements to maximize the optical power of the predetermined wavelength portion of the collimated output beam.